Young guinea-pigs were successfully protected against the effects of lethal irradiation by injections of bone marrow which was in the early stages of final regeneration after sub-lethal irradiation. In the transfused marrow 70-80% of the cells are mononuclear forms, the majority being small or medium-sized lymphocytes. The remainder are lymphocyte-like cells having features and are able to proliferate; some are very small, resembling micromyeloblasts or hematogones. The transfused marrow is excessively fatty and contains very few granulocytic and erythroid cells, their early forms being particularly scarce. After transfusion, blood neutrophil and RBC levels recovered promptly but restoration of blood lymphocytes was surprisingly slow despite the composition of the transfused marrow.